In 1908 he was appointed Lecturer in Mathematics at the University of Nottingham. At that time there was no separate department of mathematics, and Piaggio worked under W H Heaton, Professor of Physics and Mathematics. In 1919 a separate chair of mathematics was created, and Piaggio was appointed as the first Professor of Mathematics at the College. He held this position until 1950. From 1944 to 1947 he was also Dean of the Faculty of Pure Science. He helped in the administration which led to the establishment of the University of Nottingham in 1948. For some years he was a member of the Council of the Mathematical Association; he joined in 1912, and was a life member. He evolved various original ideas on the organisation of chess tournaments, and with the help of his sister he ably promoted such activities as tennis for his staff and students. He was made Professor Emeritus in 1951 and settled down happily at his home in the Park, where he was faithfully looked after by his sister, who was a woman of considerable intellectual ability, and after her death in 1957 he engaged a housekeeper. After a short illness he died at the Cedars Home on 26 June 1967.
These are the main facts of Piaggio's life. To appreciate his work we must return to the early years of the century. He was then making a profound study of differential equations and invariants. This fitted him admirably to master relativity, and by about 1920 he was considered one of the very few men in the world who really understood Einstein's ideas. Einstein himself came to Nottingham in 1929 to lecture. The blackboard on which he wrote was subsequently varnished and preserved for all to see. Naturally he and Piaggio had much in common, and Piaggio fortunately had a good knowledge of German as well as French.
Turning back again, we have Piaggio's one and only book, on differential equations, first published in 1920, but revised and enlarged in 1928. This work alone would be sufficient to establish his reputation. It has been reprinted many times and translated into many languages.
Apart from this, Piaggio wrote many articles for The Mathematical Gazette, the Journal of the London Mathematical Society, Nature and other scientific journals. The wide range of these articles is very remarkable. They deal, among many other topics, with the application of mathematics to psychology (one of his special interests), optics, air navigation, indeterminism, relativity, probability, and of course differential equations. He also reviewed a large number of books in English, French and German, including some by such eminent men as N Bohr. In addition to all this, he carried on an extensive correspondence with a number of his former students.
So much for Piaggio's erudition, capability and external activities. His teaching was a model of clarity, and he was ready at any time to take over the teaching of any of the many subjects needed by his students, and he corrected their work with meticulous care. His writing was very neat and legible: it never degenerated into a scribble.
As to his character, I think his sister-in-law's words are an admirable summary: "He was such a kind, gentle, man and sincere in all that he did." He always gave full credit to others for original ideas. Although full of energy which suggested the nickname High Tension Henry, yet he was shy and retiring and avoided social engagements. He never married. Most of his friends belonged to the scientific world or were keen on chess, which game was his major hobby. He was small in stature, with a pleasant, smiling face. Weak eyesight, which afflicted him from youth, prevented him from taking much active part in games such as tennis and cricket, which he enjoyed watching. After he retired he made a habit of taking a long walk most days. He had a vast fund of anecdotes which he used effectively in and out of class. After retirement he suffered from a period of depression, from which fortunately he soon recovered.
I feel that his life, sustained throughout by so many interests, was essentially happy, even though he was obliged for long years to work hard at Latin and Greek, in which he had no real interest, and to the study of which he attributed his weakness of sight. He was undoubtedly a great mathematician, teacher and sincere friend to all with whom he came in contact.
D A Young